What's often missed is the National Insurance number figures also
released by the Department for Work & Pensions at the same
time. Those figures tell you the number of adults from overseas
who were given permission to work.

The migration figures from the ONS show 636,000 people
coming into the UK over the year. But the NINo figures show an
amazing 917,000 people who were
registered in the same period of time. Because of the way the DWP
calculations have changed, that's likely to be an overestimate —
see below for more detail — but unlike the ONS figures,
it only covers adults.

The breakdown shows that half of the 917,000 adult overseas
workers registered come from just five countries, all of which
are in the European Union. Here's the breakdown:

Romania: 169,751

Poland: 128,448

Italy: 64,399

Spain: 58,623

Bulgaria: 44,076

There's also a huge regional gap in terms of where the migrants
register for NINos. London and the South East of England make up
more than half the registrations, while the capital alone counts
for 41% of them:

DWP

Here's how the number of NINo registrations looks over time:

DWP

But the DWP issued a warning to go with those figures,
explaining why the current ones are likely due to an
overestimate:

Although figures do accurately reflect the number of NINos
registered over time, a change to the process of recording NINos
in 2014, means that the volume of NINo registrations were lower
in the year to June 2014 and then higher in the latest year to
June 2015 than would otherwise be the case.

There were 565,719 NI numbers issued to overseas nationals
in the year to June 2014 against 917,000 in the year to June
2015, so it seems the first number is a little low and the second
a little high.